afs: Fix dynamic root lookup DNS check

In the afs dynamic root directory, the ->lookup() function does a DNS check
on the cell being asked for and if the DNS upcall reports an error it will
report an error back to userspace (typically ENOENT).

However, if a failed DNS upcall returns a new-style result, it will return
a valid result, with the status field set appropriately to indicate the
type of failure - and in that case, dns_query() doesn't return an error and
we let stat() complete with no error - which can cause confusion in
userspace as subsequent calls that trigger d_automount then fail with
ENOENT.

Fix this by checking the status result from a valid dns_query() and
returning an error if it indicates a failure.

Fixes: bbb4c4323a4d ("dns: Allow the dns resolver to retrieve a server set")
Reported-by: Markus Suvanto <markus.suvanto@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=216637
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Markus Suvanto <markus.suvanto@gmail.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 34474a0616546bb759c78089c9c51b4ebcbaa537..1f656005018ea07db18ed3944548ed0424dbab67 100644 (file)
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -114,6 +114,7 @@ static int afs_probe_cell_name(struct dentry *dentry)
        struct afs_net *net = afs_d2net(dentry);
        const char *name = dentry->d_name.name;
        size_t len = dentry->d_name.len;
+       char *result = NULL;
        int ret;
 
        /* Names prefixed with a dot are R/W mounts. */
@@ -131,9 +132,22 @@ static int afs_probe_cell_name(struct dentry *dentry)
        }
 
        ret = dns_query(net->net, "afsdb", name, len, "srv=1",
-                       NULL, NULL, false);
-       if (ret == -ENODATA || ret == -ENOKEY)
+                       &result, NULL, false);
+       if (ret == -ENODATA || ret == -ENOKEY || ret == 0)
                ret = -ENOENT;
+       if (ret > 0 && ret >= sizeof(struct dns_server_list_v1_header)) {
+               struct dns_server_list_v1_header *v1 = (void *)result;
+
+               if (v1->hdr.zero == 0 &&
+                   v1->hdr.content == DNS_PAYLOAD_IS_SERVER_LIST &&
+                   v1->hdr.version == 1 &&
+                   (v1->status != DNS_LOOKUP_GOOD &&
+                    v1->status != DNS_LOOKUP_GOOD_WITH_BAD))
+                       return -ENOENT;
+
+       }
+
+       kfree(result);
        return ret;
 }